The present invention relates to the degassing and filtration of molten metal, and particularly to the treatment of said metal with a fluxing gas.
Molten metal, particularly molten aluminum, in practice generally contains entrained and dissolved impurities, both gaseous and solid, which are deleterious to the final cast product. These impurities may affect the final cast product after the molten metal is solidified, whereby processing may be hampered or the final product may be less ductile, or have poor finishing and anodizing characteristics. The impurities may originate from several sources. For example, the impurities may include metallic impurities such as alkaline and alkaline earth metals, and occluded hydrogen gas and dissolved surface oxide films which have become broken up and are entrained in the molten metal. In addition, the inclusions may originate as insoluble impurities, such as carbides, borides and others or eroded furnace and trough refractories.
It is naturally highly desirable to improve the degassing and filtration of molten metals in order to remove or minimize such impurities in the final cast product, particularly with respect to molten aluminum and especially, for example, when the resultant metal is to be used in a decorative product, such as decorative trim or sheet, or products bearing critical specifications, such as aircraft forgings and extrusions, and light gauge foil stock. Impurities as aforesaid cause loss of properties such as tensile strength and corrosion resistance in the final solidified alloy and lead to degradation of processing efficiency and loss of properties in the final product. For example, one type of finishing flaw which is particularly significant in decorative trim or sheet is a stringer defect known as a linear defect.
Conventionally conducted gas fluxing processes, such as general hearth fluxing, have involved the introduction of the fluxing gas to a melting or holding furnace containing a quantity of molten metal. This procedure required that the furnace be shut down while the fluxing gas is circulated, so that the metal being treated would remain constant and treatment could take place. This procedure had many drawbacks, among them, the reduced efficiency resulting from the prolonged idleness of the furnace during fluxing, as well as the lack of efficiency due to the low surface area to volume ratio between the gas flux and the molten metal. Further factors comprised the restriction of location to the furnace which permitted the re-entry of impurities to the melt before casting, and the high emissions resulting from both the sheer quantity of flux required and the location of its circulation.
As an alternative to the batch-type fluxing operations employed as set out above, certain fluxing operations were employed in an in-line manner; that is, the operation and associated apparatus were located outside the melting or holding furnace and often between the melting furnace and either the holding furnace or the holding furnace and the casting station. This helped to alleviate the inefficiency caused by furnace shut-down, but was not as successful in improving the efficiency of the operation itself, in that undesirably large quantities of fluxing gas were often required per unit of molten metal, which was both costly and detrimental to air purity. Some of these processes utilized in conjunction therewith conventional filter media such as open-weave glass cloth screens, and bed filters made up, for example, of tabular alumina, which both exhibit as a primary disadvantage the inability to maintain a uniform pore size.
Porous ceramic foam materials are known in the art, for example, having been described in U.S. Pat. Nos. 3,090,094 and 3,097,930. These porous ceramic foam materials are known to be particularly useful in filtering molten metal, as described in U.S. Pat. No. 3,893,917 for "Molten Metal Filter" by Michael J. Pryor and Thomas J. Gray, patented July 8, 1975, and also as described in our copending U.S. Pat. application Ser. No. 563,213 for "Ceramic Foam Filter," filed Mar. 28, 1975. Further, an apparatus and method for the filtration of molten metals which employs filter plates prepared from said porous ceramic materials is described in our copending parent application Ser. No. 597,963.
Porous ceramic foam materials are particularly useful for filtering molten metal for a variety of reasons included among which are their excellent filtration efficiency resulting in part from their uniform, controllable pore size, and low cost, as well as ease of use and replaceability. The fact that these ceramic foam filters are convenient and inexpensive to prepare and use has prompted the development of means for easily and conveniently employing these porous, molten metal filters in an in-line degassing and filtration unit which provides a highly efficient assembly.
Accordingly, it is a principal object of the present invention to provide an improved method and apparatus for the degassing and filtration of molten metal which employs filter-type plates of uniform porosity.
It is a particular object of the present invention to provide an apparatus and method as aforesaid which achieves improved degassing by counter-current contact between molten metal and fluxing gas taking place within and above one of the filter-type plates.
It is a still further object of the present invention to provide improvements as aforesaid which are convenient and inexpensive to utilize and which result in highly efficient metal degassing and filtration.
Further objects and advantages of the present invention will appear hereinbelow.